Integrated heater on mems cap for wafer scale packaged mems sensors

ABSTRACT

A system and method for controlling temperature of a MEMS sensor are disclosed. In a first aspect, the system comprises a MEMS cap encapsulating the MEMS sensor and a CMOS die vertically arranged to the MEMS cap. The system includes a heater integrated into the MEMS cap. The integrated heater is activated to control the temperature of the MEMS sensor. In a second aspect, the method comprises encapsulating the MEMS sensor with a MEMS cap and coupling a CMOS die to the MEMS cap. The method includes integrating a heater into the MEMS cap. The integrated heater is activated to control the temperature of the MEMS sensor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 61/502,643, filed on Jun. 29, 2011, entitled “INTEGRATEDHEATER ON MEMS CAP FOR WAFER SCALE PACKAGED MEMS SENSORS,” which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to Microelectromechanical Systems (MEMS)sensors, and more particularly, to a MEMS sensor that requires rapidheating for temperature characterization, calibration, and compensation.

BACKGROUND

MEMS sensors (e.g. accelerometers, gyros, compasses, pressure sensors,oscillators, etc.) require temperature compensation to reduce outputsignal changes that result from temperature variations. Conventionalmethods of temperature compensation are accomplished during productionby measuring the output signal of each sensor at known temperatures,determining the temperature dependence of the output signals, andremoving the effect of temperature variations by appropriate on chip oroff line signal processing. In either case, the temperature dependencedata is stored in a non-volatile memory inside the MEMS sensor.

The drawbacks of these conventional methods include the inefficient andtime consuming process of establishing the response of the output signalto temperature variations by applying a known temperature to the MEMSsensor and measuring the resultant output signals. For high volumeproduction, external heating of the MEMS sensor die is prohibitive dueto lengthy heating time and complex test setup.

Therefore, there is a strong need for a cost-effective solution thatovercomes the above issues by enabling fast and real-time temperaturecompensation that is achieved through an integrated heater. The presentinvention addresses such a need.

SUMMARY OF THE INVENTION

A system and method for controlling temperature of a MEMS sensor aredisclosed. In a first aspect, the system comprises a MEMS capencapsulating the MEMS sensor and a CMOS die coupled to the MEMS cap.The system includes a heater integrated into the MEMS cap. Theintegrated heater is activated to control the temperature of the MEMSsensor.

In a second aspect, the method comprises encapsulating the MEMS sensorwith a MEMS cap and coupling a CMOS die to the MEMS cap. The methodincludes integrating a heater into the MEMS cap. The integrated heateris activated to control the temperature of the MEMS sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures illustrate several embodiments of the inventionand, together with the description, serve to explain the principles ofthe invention. One of ordinary skill in the art will recognize that theparticular embodiments illustrated in the figures are merely exemplary,and are not intended to limit the scope of the present invention.

FIG. 1 illustrates a system for controlling temperature of a MEMS sensorin accordance with a first embodiment.

FIG. 2 illustrates a system for controlling temperature of a MEMS sensorin accordance with a second embodiment.

FIG. 3 illustrates a system for controlling temperature of a MEMS sensorin accordance with a third embodiment.

FIG. 4 illustrates a system for controlling temperature of a MEMS sensorin accordance with a fourth embodiment.

FIG. 5 illustrates a system for controlling temperature of a MEMS sensorin accordance with a fifth embodiment.

FIG. 6 illustrates a diagram showing a temperature rise in a CMOS die inaccordance with an embodiment.

DETAILED DESCRIPTION

The present invention relates to Microelectromechanical Systems (MEMS)sensors, and more particularly, to a MEMS sensor that requires rapidheating for temperature characterization, calibration, and compensation.The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments and the generic principles and featuresdescribed herein will be readily apparent to those skilled in the art.Thus, the present invention is not intended to be limited to theembodiments shown but is to be accorded the widest scope consistent withthe principles and features described herein.

A system and method in accordance with the present invention allows forquickly changing the temperature of a fabricated apparatus comprising aMEMS cap, a MEMS sensor, and/or a CMOS die to avoid inefficient externalheating. By integrating a heater into the MEMS cap that encapsulates theMEMS sensor and controlling a current through the integrated heater byan external source or by the CMOS die, Joule heating causes thetemperature of the heater to increase thereby increasing the temperatureof the various components of the apparatus comprising the MEMS cap, theMEMS sensor, and/or the CMOS die. One of ordinary skill in the artreadily recognizes that the apparatus can be fabricated using a varietyof methodologies including but not limited to wafer scale packagingusing the Nasiri Fabrication (NF) platform.

Due to the high thermal conductivity of the silicon of the MEMS cap,MEMS sensor, and CMOS die and their small masses, the time constant orthe time to heat up the masses is very small thereby enabling rapidheating of the whole apparatus. This rapid heating enables moreefficient testing and calibration processes of the whole MEMS apparatusto be conducted.

To describe the features of the present invention in more detail, refernow to the following description in conjunction with the accompanyingFigures.

FIG. 1 illustrates a system 100 for controlling temperature of a MEMSsensor in accordance with a first embodiment. The system 100 includes aCMOS die 102, a MEMS cap 104 coupled to the CMOS die 102, and a heater106 integrated into the MEMS cap 104. The MEMS cap 104 encapsulates theMEMS sensor. In one embodiment, the heater 106 is integrated into a topsurface of the MEMS cap 104. By activating and/or deactivating theheater 106, the temperature of any of the MEMS cap 104, encapsulatedMEMS sensor, and the CMOS die 102 can be increased and/or decreasedrespectively.

In one embodiment, the heater 106 is activated by applying a currentthrough two terminals (not shown in FIG. 1) of the heater 106. Once theheater 106 is activated, the temperature of the heater 106 is increaseddue to Joule heating which in turn increases the temperature of the MEMScap 104 and the CMOS die 102 coupled to the MEMS cap 104. The highthermal conductivity of the silicon components of the system 100 and thesmall masses involved in the system 100 results in a time constant thatis very small including but not limited to the order of milliseconds.Accordingly, the system 100 is rapidly heated by the heater 106.

One of ordinary skill in the art readily recognizes that the heater 106can be integrated into the MEMS cap 104 during production of the system100 or post-production by an end-user and that would be within thespirit and scope of the present invention. In one embodiment, the MEMScap 104 is made of a variety of materials including but not limited tosilicon.

In one embodiment, the heater 106 is a resistive heater with terminalsconnected to either the CMOS die 102 via the MEMS cap 104 or directly topackage pads. In this embodiment, the two terminal heater is integratedwith the MEMS caps to allow current to pass across the two terminals toincrease the temperature. FIG. 2 illustrates a system 200 forcontrolling temperature of a MEMS sensor in accordance with a secondembodiment. The system 200 includes a CMOS die 102, a MEMS cap 104coupled to the CMOS die 102, a heater 106 integrated into the MEMS cap104, an outer package 208 housing the CMOS die 102, the MEMS cap 104,and the heater 106, and at least one package pad 210 connected to theassembly of the MEMS cap 104/CMOS die 102 structure via at least onewire bond 212.

By running a current through the heater 106, the heater 106 is activatedto control the temperature of the system 200. The activation of theheater 106 is controlled by the CMOS die 102. In one embodiment, theCMOS die 102 includes a temperature sensor, an electronic circuit formeasuring the temperature sensor's output signals, an electronic circuitfor measuring the MEMS sensor's output signals, and an electroniccircuit for energizing and/or activating the heater 106. In oneembodiment, the on chip temperature sensor measures the temperature ofthe system 200. In another embodiment, an external thermocouple that isin contact with the top of the outer package 208 measures thetemperature of the system 200.

One of ordinary skill in the art readily recognizes that the CMOS die102 can include a variety of different types of electronic circuitcomponents and that would be within the spirit and scope of the presentinvention. In another embodiment, the activation of the heater 106 iscontrolled by an external source. There are different ways to integratethe resistive heater into the MEMS cap including but not limited tomicromachining and screen printing methodologies. In one embodiment, theresistive heater is an aluminum film deposited into a top surface of theMEMS cap. In this embodiment, the resistive heater is any material thatconducts or semi-conducts current including but not limited topolysilicon, various metals, various metallic silicides, and othersilicon based films, and resistive patches. In another embodiment, theresistive heater is a high resistance material including but not limitedto a polymer with metal dust.

FIG. 3 illustrates a system 300 for controlling temperature of a MEMSsensor in accordance with a third embodiment. The system 300 includes aCMOS die 102, a silicon MEMS cap 304 coupled to the CMOS die 102, analuminum film heater 306 integrated into the silicon MEMS cap 304, afirst wire bond 308 coupled to a first terminal of the aluminum filmheater 306, and a second wire bond 310 coupled to a second terminal ofthe aluminum film heater 306.

In one embodiment, the aluminum film heater 306 is deposited into a topsurface of the silicon MEMS cap 304. In this embodiment, the aluminumfilm heater 306 is split into two discontinuous pieces by a cut 320 ofvarying degree including but not limited to a shallow cut and a deepcut. The cut provides a discontinuity in the aluminum film. The currentpath is then subject to pass through a portion of the silicon MEMS cap304 in the region of the area of discontinuity. The resistance betweenthe first and second heater terminals is determined by the doping levelof the silicon MEMS cap 304. In one embodiment, the aluminum film heater306 is deposited after implantation of the silicon MEMS cap 304 to getgood contact between the deposited aluminum film heater 306 and thesilicon MEMS cap 304.

One of ordinary skill in the art readily recognizes that between thefirst and second heater terminals, a variety of resistances and voltagedifferences can be utilized to determine the amount of power generatedfor heating of the aluminum film heater 306 and that would be within thespirit and scope of the present invention. Additionally, one of ordinaryskill in the art readily recognizes that a variety of outer packagemasses and heating times can be utilized and that would be within thespirit and scope of the present invention.

In one embodiment, a resistance of 100 ohms and a 10 volt (V) voltagedifference between the first and second heater terminals is assumed.Combining the equations Power (P)=Current (I)×Voltage (V) andI=V/Resistance (R), results in P=V²/R or 1 Watt of power being generatedfor the heating of the aluminum film heater 306. Additionally, in thisembodiment, if the outer package housing the CMOS die 102, the siliconMEMS cap 304, and the aluminum film heater 306, is assumed to be mostlyplastic with a specific heat of 1.2 Joules/Grams×Degree Celsius (J/g*C)and a total mass of 30 milligrams (mg), 5 seconds of heating thealuminum film heater 306 will result in a 140 degree C. temperatureincrease being achieved.

To alter the resistance between the first and second heater terminalsand thus the duration required to heat the MEMS cap and/or CMOS die,additional cuts can be made into the heater that is integrated into thesilicon MEMS cap. FIG. 4 illustrates a system 400 for controllingtemperature of a MEMS sensor in accordance with a fourth embodiment. Thesystem 400 resembles the system 300 of FIG. 3 and includes a CMOS die102, a silicon MEMS cap 404 coupled to the CMOS die 102, a heater 406integrated into the silicon MEMS cap 404, a first wire bond 408 coupledto a first terminal of the heater 406, and a second wire bond 410coupled to a second terminal of the heater 406.

In one embodiment, the heater 406 integrated into the silicon MEMS cap404 includes two cuts. One of ordinary skill in the art readilyrecognizes that a plurality of cuts can be utilized and that would bewithin the spirit and scope of the present invention. Additionally, theheater 406 resistance is maintained at a higher level than the otherresistances along the current path so that enough power is dissipatedacross the heater 406 and external contacts and cables do not get hotterthan the heater 406. The power dissipated by the heater 406 is I²×R. Ifthe heater 406 resistance is too small (e.g. <1 ohm), then heatingcircuitry could supply a current for Joule heating rather than supplyinga voltage.

FIG. 5 illustrates a system 500 for controlling temperature of a MEMSsensor in accordance with a fifth embodiment. The system 500 includes aCMOS die 102, a MEMS cap 504 coupled to the CMOS die 102, a heater 506integrated into the MEMS cap 504, a first wire bond 508 coupled to afirst terminal of the heater 506, and a second wire bond 510 coupled toa second terminal of the heater 506. In one embodiment, the heater 506comprises aluminum layers lithographically patterned on a top surface ofthe MEMS cap 504.

By lithographically patterning the aluminum layers, a resistive heaterwith desired resistance is attained. In one embodiment, the top surfacearea of the MEMS cap 504 is 2 millimeters (mm) by 1 mm and 10 segmentsof 1 mm long aluminum layers are lithographically patterned on the topsurface of the MEMS cap 504. In this embodiment, assuming theresistivity of aluminum is 2.81 ohm×meter and the aluminum layers have 4micron widths and 1 micro thicknesses, the resistance of the heater 506is 70 ohms.

FIG. 6 illustrates a diagram 600 showing a temperature rise in a CMOSdie in accordance with an embodiment. The temperature rise is measuredby a temperature sensor within CMOS die circuitry or coupled externallyto the CMOS die. In the diagram 600, the resistance of the heaterintegrated into the MEMS cap is adjusted to be 200 ohms and the verticalscale is 1 Celsius (C)=280 least significant bits (LSB). When anexternal power supply of 10 V is connected to the heater, thetemperature of the CMOS die increases rapidly and within 5 seconds, anapproximately 36 degree Celsius temperature rise is achieved.

As above described, the system and method allow for rapidly controllingtemperature compensation of wafer scale packaged MEMS sensors to moreefficiently and more accurately measure and calibrate component signaloutputs of the whole MEMS apparatus. By integrating a heater into theMEMS cap of the wafer scale packaged MEMS sensor, temperaturecompensation in volume production of MEMS sensors can achievetemperature rises in approximately one second without the usage ofcomplicated equipment. In comparison, conventional methods typicallyrequire approximately 20-30 seconds for the temperature rises and usecomplicated equipment such as well isolated ovens and/or contactheaters.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

1. A system for controlling temperature of a MEMS sensor, the systemcomprising: a MEMS cap encapsulating the MEMS sensor; a CMOS dievertically arranged to the MEMS cap; and a heater integrated into theMEMS cap, wherein the integrated heater is activated to control thetemperature of the MEMS sensor.
 2. The system of claim 1, furthercomprising: a wafer scale package fabricated to encapsulate the MEMSsensor, the MEMS cap with integrated heater, and the CMOS die.
 3. Thesystem of claim 2, wherein terminals of the integrated heater areconnected directly to at least one pad of the wafer scale package via atleast one wire bond.
 4. The system of claim 1, wherein terminals of theintegrated heater are connected to the CMOS die via the MEMS sensor. 5.The system of claim 1, wherein the MEMS cap is a silicon MEMS cap andthe integrated heater is a bulk silicon single crystal resistiveelement.
 6. The system of claim 1, wherein a current applied to activatethe integrated heater is controlled by any of an external source and theCMOS die.
 7. The system of claim 1, wherein the integrated heatercomprises: an aluminum deposit on a top surface of the MEMS cap, whereinthe MEMS cap is cut to separate the aluminum deposit into twodiscontinuous pieces.
 8. The system of claim 1, wherein the integratedheater comprises: an aluminum deposit on a top surface of the MEMS cap,wherein the MEMS cap is cut to separate the aluminum deposit into aplurality of discontinuous pieces.
 9. The system of claim 1, wherein theintegrated heater comprises: a lithographically patterned aluminumdeposit on a top surface of the MEMS cap to implement a desiredresistance.
 10. The system of claim 1, wherein the integrated heatercomprises: a high resistance material deposit on a top surface of theMEMS cap.
 11. The system of claim 10, wherein the high resistancematerial deposit is a polymer with metal dust.
 12. A method forcontrolling temperature of a MEMS sensor, the method comprising:encapsulating the MEMS sensor with a MEMS cap; coupling a CMOS die tothe MEMS cap; integrating a heater into the MEMS cap; and activating theintegrated heater to control the temperature of the MEMS sensor.
 13. Themethod of claim 12, further comprising: fabricating a wafer scalepackage to encapsulate the MEMS sensor, the MEMS cap with integratedheater, and the CMOS die.
 14. The method of claim 13, furthercomprising: connecting terminals of the integrated heater to at leastone pad of the wafer scale package via at least one wire bond
 15. Themethod of claim 12, further comprising: connecting terminals of theintegrated heater to the CMOS die via the MEMS sensor.
 16. The method ofclaim 12, further comprising: controlling a current applied to activatethe integrated heater by any of an external source and the CMOS die. 17.The method of claim 12, wherein the integrated heater is an aluminumdeposit on a top surface of the MEMS cap, further comprising: cuttingthe MEMS cap to separate the aluminum deposit into two discontinuouspieces.
 18. The method of claim 12, wherein the integrated heater is analuminum deposit on a top surface of the MEMS cap, further comprising:cutting the MEMS cap to separate the aluminum deposit into a pluralityof discontinuous pieces.
 19. The method of claim 12, wherein theintegrated heater is an aluminum deposit on a top surface of the MEMScap, further comprising: lithographically patterning the aluminumdeposit on the top surface of the MEMS cap to implement a desiredresistance.
 20. The method of claim 13, further comprising: measuring atemperature of the wafer scale package using any of a chip sensor of theCMOS die and an external thermocouple in contact with the wafer scalepackage.